The internal resistance of a battery indicates the capacity of the battery to supply power to a load or circuit. The internal resistance may be measured periodically to insure that a battery meets a predetermined state of health (SOH). Based on field testing of various types of batteries, such as lead, lead acid and lead calcium batteries, once the internal resistance increases to more than 25% above its nominal value, the battery is unable to meet its capacity requirements and fails capacity tests.
Referring now to FIG. 1, a schematic model is shown of battery resistance. The model includes a natural capacitance XC, electromechanical resistance RE, and metallic resistance RM. Metallic resistance RM is in series with a parallel combination of the electromechanical resistance RE and natural capacitance XC.
Electrochemical resistance RE represents the internal resistance of the battery and includes a series combination of resistances RPASTE, RELECTROLYTE, and RSEPERATOR. RPASTE represents a resistance that is presented by cell paste used on metallic grids of the battery. RELECTROLYTE represents a resistance of electrolytes in the battery. RSEPARATOR represents a resistance of the separators in the battery.
Metallic resistance RM includes a series combination of resistances RGRID TO POST, RGRID, RSTRAP, RTERMINAL POST. RGRID TO POST represents a resistance presented by a junction resistance between a battery post and a metallic grid that connects a plurality of battery cells. RGRID represents a resistance presented by the metallic grid. RSTRAP represents a resistance presented by a conducting bar or wire that connects the battery post to a post of another battery. RPOST represents a resistance presented by the battery post.
Referring now to FIG. 2, an oscilloscope trace shows an example of battery voltage during an internal resistance test that is performed in accordance with the prior art. The battery voltage is represented by trace 20. A horizontal axis 22 represents time at 100 mS per division. Prior to time 24, the battery voltage is at a float voltage. Float voltage is the battery voltage when the battery is fully charged and unloaded.
At time 24, an electrical load is applied to the battery. The battery voltage drops exponentially to a loaded voltage at time 26. The battery internal resistance can be estimated by ΔV/I, where ΔV is the difference between the unloaded voltage and the loaded voltage, and I is the battery current. After time 26 the load is removed and the battery voltage recovers to the float voltage. FIG. 2 shows that each iteration of the battery resistance test can take about 400 mS.